The evidence that mammals are capable of detecting geomagnetic field inversions and magnitude changes has made important strides in the past five years. However, the anatomy and physiology of magnetosensitivity in mammals remains only incompletely understood. Recent studies suggest that the retinae may be the magnetoreceptor and the pineal gland may be a component of the magnetosensitivity system. The proposed investigations seek to clarify what in the magnetic field is perceived, to what extent retinal photoreceptors are magnetoreceptors and whether variations in the geomagnetic field can be utilized for entrainment of circadian activity rhythms in rodents. Retinal and pineal gland melatonin synthesis will be assessed by means of indoleamine HPLC with electrochemical detection and radioenzymatic assays for the relevant enzymes. Retinal dopamine and its metabolites will be assayed similarly. Both parameters are sensitive to another environmental variable, namely light. Various alterations of the geomagnetic field will be induced in order to determine the primary features of magnetosensitivity. Experiments with constant light-induced photoreceptor degeneration and inherited retinal dystrophy will elucidate the role played by the photoreceptor in magnetic field detection. Behavioral studies under constant conditions, in which brief exposure of free-running animals to inversions of the geomagnetic field will be used to test for entrainment, should clarify a potential role for magnetic information. These investigations should contribute new insights into the manner by which subtle geomagnetic variations are detected and utilized by the mammalian organism.